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Putting some bits and pieces together

So what's the point of a voltage regulator, a police light at 12V, a one touch switch at 6V, an isolating optocoupler, a Darlington Transistor and a police siren (also at 6V)?

Well, you could put them all together in one symphonic circuit to simulate a regular Saturday night in town. You see the flashing lights first - you think they're for someone else - then the siren starts!

I had an idea to use only a single power source for the whole circuit. The maximum voltage needed is 12V (makes sense - it's police car voltage!) but the one touch switch and the siren both need only 6V. Hence I used the LM317 regulator with a 220Ω resistor and a 1kΩ potentiometer set at 836Ω to give the required voltage.

https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator

From the little breadboard with the LM317 the power splits off with 12V going to the police light board and 6V travelling to the one touch switch. The switch output does not have the power to drive the siren which seems to need anywhere from about 160mA to 240mA depending on the frequency of the output. I tried some transistor options but they weren't initially cooperating and instead of creating a new paradigm I thought why not use an optocoupler instead of the LED from the original switch circuit? It would make sense to have the optocoupler on a whole new circuit, isolated from the switch, but I thought seeing as how the whole circuit was based on a simulation, why not simulate the isolation! Er...and laziness.

Sadly the output from the optocoupler (50mA) wasn't sufficient to drive the siren, so I popped in a BC547 (110mA and hFE 200) which allowed a few stuttering pops and splutters from the siren, but not enough to scare any potential perps! 

Then the trusty favourite SS8050 (1.5A and hFE 200) swung into action forthwith which allowed a recognisable but slightly anaemic siren. Finally I remembered the wonders of Darlington Transistors and referred back to the ULN2083 Darlington Array from an earlier blog. Using a whole array on a tiny breadboard seemed a bit over the top so I settled on a TIP120 single Darlington transistor which allows 5A at a hFE of around 2500. Presto we have lights and a switchable siren. Pay no attention to the rogue transistor on the latching switch breadboard with the single blue jumper lead attached to ground - clearly I was trying a few options and forgot to "switch" it out again!